The present invention relates to orifice assemblies for fluid flow conduits and more particularly to orifice assemblies for increasing the resistance to fluid flowing through such conduits. The present invention has particular application in forced circulation steam generating units. In steam generating units constructed to operate with forced circulation, a multiplicity of steam generating tubes are connected to a common header from which they receive their supply of water thereby forming a multiplicity of parallel tube circuits in which steam is generated. The common header is connected to the discharge of a pump which receives its water from a steam and water separating drum. The tube circuits discharge the steam and water mixture into the steam and water separating drum.
In the past, orifices or other flow restricting means have been employed at the entrance of each of the multiplicity of tubes for controlling the distribution of water from the common header to the individual tube circuits. A high flow resistance is required to assure uniform flow distribution and prevention of flow reversals in shaded water wall panels and flow starvation of adjacent tubes in the event of a single tube rupture. In connection with the latter aspect of flow starvation, the orifices serve to throttle or choke the flow to the ruptured steam generating tube. This then insures that the remaining mass flow from the common header will be distributed to the remaining, nonruptured steam generating tubes.
Present day means of securing the orifices into the tubes are as follows: First a ring is welded into the inner surface of each tube. Then a plate having a suitably sized orifice therein is mechanically clamped to the ring. This type of fastening has the advantage of being able to replace the orifice plate at a later date if it is determined that the wrong sized orifice has been placed in some of the tubes, or in the event the orifice plate becomes corroded to the point of being inoperative. This type of securing orifices in tubes also has some disadvantages. Because of the requirement of welding in the ring, it is an expensive procedure. Also because the ring and its associated orifice plate is located in a horizontal component of the tubes in many instances, it is not possible to completely drain the tubes during a shutdown period, or when the tubes are given an acid-wash. Also in high pressure steam generators, such as one operating at supercritical pressure, there can be considerable leakage of fluid between the orifice plate and the welded-in ring, making the orifice ineffective in accurately throttling the flow a desired amount.